To start is fairly easy; add some white gel food coloring into a small glass mixing bowl. Slowly add a few drops at a time to the white base and between each addition, mix the colors together using a toothpick or skewer.
Between every addition and mixing of colors, or even if you think you are close to getting the right color, you can test it on a piece of paper towel. To do this, simply take the toothpick with some color on it and softly paint it on the sheet of white kitchen towel.
Once you have achieved the desired shade of grey, you can either use it immediately, or you can store it for future use. To use it, slowly add a few drops of your homemade grey food coloring to the item you want to color.